Piling on in Football can refer to one of two things: when plays jump on top of one-another to form a pile on the field, usually in an effort to gain control of a loose ball or as a celebration, or when a team continues to score even though the game is almost over and they're almost certainly going to win.
When players pile on to one-another, this is often called a dogpile. Often players pile on to one-another when trying to gain possession of a loose football. When enough players pile on so that the referees can no longer see who has possession of the football, the play is called dead to the pile may be sorted out to determine who has the football.
Piling on is widely criticized as the referees cannot control what happens inside of the pile, therefore the ball may change hands multiple times, even if the play has already been called dead. If more players jump into the pile after the play is called dead, they may be called for an unsportsmanlike conduct or unnecessary roughness penalty, to the referees discretion.
Piling on with scoring is the act of continually scoring even if the game is out of reach for the other team to win. It is considered very unsportsmanlike to pile on the scoreboard after a game is out of reach, and often fights may break out.